schoolswikiaorg-20200215-history
Kinetic energy
Energy There are two forms of fundamental energy: (1) Kinetic energy, which is energy of motion; and (2) Potential energy, whcih can be considered "stored" energy. For instance, the mechanical energy involved in picking up and dropping a ball is partly potential energy and partly kinetic energy. Pick up the ball, and you give it potential energy. Drop the ball, and the potential energy of the ball turns into kinetic energy. Units of Work and Enegy Due to the relationship between energy and work, energy is measured in units of work. In its scientific sense, work is the movement of an object against a resisting force. For example, a person does work by lifting an object. Lifting an object requires work because the lifter must overcome the force of gravity on the object. The amount of work done, and therefore the amount of energy used, equals that force multiplied by the distance the object is lifted. The kinetic energy used to lift the object is "stored" in the object as potential energy. In the inch-pound system of units customarily used in the United States, work and energy are commonly measured in foot-pounds. In the International System of Units (SI), the modern metric system, work and energy are measured in units called joules-named for James Prescott Joule. One joule equals 0.738 foot-pound. Calculating Quantities of Energy Scientists use Newtonian equations and relativistic equations to calculate quantities of energy. Newtonian equations are based on Newton's laws of motion, which were discovered by the English scientist Isaac Newton. The laws were published in 1687 in Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), a work usually called simply Principia or Principia Mathematica. Kinetic Energy The equation for kinetic energy is Ek = (1/2)mv2. In this equation, Ek is energy in joules, m is mass in kilograms, and v is velocity in meters per second. The kinetic energy of an object increases with the square of its velocity. For example, an automobile moving at 60 miles an hour has four times the energy it would have at 30 miles an hour. This increase in kinetic energy is why high-speed collisions are so much more dangerous and damaging than low-speed collisions. When a moving object collides with another object, energy and momentum are transferred. The energy given up by the moving object is called the impact. Objects moving at high velocity have high impact if they strike another object. Objects obtain momentum and kinetic energy from forces acting on them for a period of time. The longer the time of action, the larger the momentum and kinetic energy. In such sports as tennis and golf, players follow through with their rackets and clubs so that force is applied to the ball for the longest time possible. As a result, the ball travels faster and has greater momentum and kinetic energy. Sample Problem Suppose the person threw the rock at a velocity (speed in a particular direction) of 12 meters per second. What would be the kinetic energy of the rock? The equation for kinetic energy is Ek = (1/2)mv2. In this equation, Ek is energy in joules, m is mass in kilograms, and v is velocity in meters per second. The 2 beside the v indicates that v is to be squared (multiplied by itself). So the calculation is Ek = (1/2)(2)(12)(12) = 144 joules.